Process to separate the vanadium contained in inorganic acid solutions

ABSTRACT

A process for recovering vanadium contained in inorganic acid solutions by precipitating the vanadium as a solid compound of vanadium and alkali metal or monovalent cation ferricyanide. Separation is carried out electrochemically by depositing the compound on to a metal immersed in the acid solution that contains vanadium, to which a ferricyanide salt of an alkali metal or a monovalent cation has been added. If the inorganic acid present in solution is different from nitric acid, the vanadium can be also separated by direct addition of a ferricyanide salt of an alkali metal or a monovalent cation to the acid solution containing vanadium. The method described allows recovery of vanadium without modifying the initial composition of the solution, except for the concentration of the vanadium dissolved.

SUMMARY

[0001] The invention refers to a chemical process that recovers the vanadium contained in inorganic acid solutions, precipitating it as a solid compound of vanadium and alkaline metal or monovalent cation ferrocyanide. Separation is carried out electrochemically, depositing the compound on to a metal immersed in the acid solution that contains vanadium as well as other dissolved metals, to which a ferrocyanide salt of an alkaline metal or a monovalent cation has been previously added. If the inorganic acid present in solution is different from nitric acid, the vanadium can be also separated by direct addition of a ferrocyanide salt of an alkaline metal or a monovalent cation to the acid solution containing vanadium. The method described allows recovery of vanadium without modifying the initial composition of the solution, except for the concentration of the vanadium dissolved.

TECHNICAL BACKGROUND

[0002] A method to separate dissolved vanadium in acid solutions is presented here. These solutions or liquors may be produced, for example, during treatments for the demetallization of carbonaceous materials, such as fractions of heavy crude oils, or from residues obtained during oil refining processes, for example petroleum coke. The typical vanadium content in these liquors usually exceeds 4%, thus representing a significant source of this metal. These solutions usually contain other metals such as nickel, in concentration around 0,1%, as well as iron, although in lesser quantities.

[0003] The recovery of vanadium from these solutions is of interest, because of the diverse industrial applications of this metal, such as catalyst in oil refining processes, the manufacture of ferrous alloys (steels) and the construction of batteries.

[0004] Processes reported in the literature for the recovery of dissolved vanadium from inorganic acid solutions involve essentially precipitation with complexing agents such as ammonia to obtain ammonium metavanadate.

[0005] These procedures generally require adjustment of the solution pH from its initial value, usually between zero and one, to a value close to two. The latter implies partial neutralization of the original acid content, meaning that the neutralized solution must be thrown away, which is not desirable.

[0006] If the acid concentration can be maintained during recovery of the metal, as resulting from the methodology presented here, then the liquor can be used again in the initial process of demetallization. In this way, the cost associated to the overall process is reduced, and also production of waste materials that may involve environmental hazard is reduced.

[0007] The process described here allows recovery of vanadium from solution, whatever its oxidation state, without changing the initial composition of the solution, except for the vanadium content, which decreases in about 99%. In this way it is possible to reuse the liquor for the demetallization of carbanaceous materials, as mentioned in the previous paragraph, representing advantages with respect to techniques hitherto proposed in the literature.

[0008] In the process mentioned here, the recovery is carried out in a single step, and results in a ferrocyanide compound of vanadium and monovalent cation, which has practical applications in electronic devices, such as electrochromic screens. In procedures appearing in the literature, the compound formed to precipitate dissolved vanadium is ammonium metavanadate. This requires an additional step to transform it into vanadium pentoxide; this compound has known practical application and considerable commercial value. Conversion is attained heating the ammonium metavanadate in air at a temperature higher than 650 degrees Celsius.

DESCRIPTION OF THE INVENTION

[0009] The procedure consists in initially adding to the acid solution that contains the dissolved vanadium a ferricyanide salt (hexacyanoferrate (III)) of a metal of the alkaline group in the periodic table, like lithium, sodium, potassium, etc., or any other monovalent cation, i.e. with a single positive charge, for example NH₄ ⁺.

[0010] The acid solutions or liquors can originate, for example, from demetallization treatments of carbonaceous materials, like fractions of heavy crude oil or residues obtained from oil refining processes (petroleum coke, for example).

[0011] The acid solution or liquor that contains the vanadium may be composed of any of the following acids: sulfuric, nitric, perchloric, hydrochloric, phosphoric or hydrofluoric, or may consist of a mixture of these. They may be either concentrated or dilute solutions of the acids, as well as concentrated or diluted with respect to vanadium.

[0012] The amount of salt that is necessary to add is estimated from the approximate concentration of vanadium to be recovered in the original solution. This should be at least two moles of iron in the added salt for each three moles of dissolved vanadium to be recovered from solution. An excess of the iron compound may be added.

[0013] Hereafter, a piece of conducting material, which may be metal or carbon, is immersed in the solution. Then a constant cathodic current density higher than 0.52 mA/cm² is applied.

[0014] It is also possible to conduct the electrodeposition applying a constant potential more negative than 0.85 V with respect to the saturated calomel electrode.

[0015] The charge needed for electrodeposition of all the dissolved material is determined from the vanadium concentration present in the solution or liquor and to be recovered, considering that is necessary to pass approximately 36.81 C/cm² for each gram of vanadium recovered as deposit.

[0016] Deposition initially occurs on the metallic surface contacting the aqueous solution. When the surface has been totally covered, then the compound continues to deposit on to the adhered material, and eventually the excess solid detaches from the metallic piece and disperses as a powder in the solution.

[0017] Once the passing of the estimated charge for recovery of vanadium in solution has been completed, the resulting solid compound suspended in the solution is then separated using a physical method, consisting of filtration, centrifugation, etc. Because of its low adherence, the material deposited on the conducting material is recovered by soft scraping of the surface.

[0018] With this procedure, 99% of the vanadium in solution can be separated without any interference from other dissolved materials.

[0019] A particular case occurs when sulfuric acid is the sole inorganic acid present in solution or liquor. In this case the dissolved vanadium can be recovered also chemically, i.e., without applying any electrical charge, in the form of vanadyl and alkaline metal or monovalent cation ferrocyanide. To accomplish this, a known quantity of ferrocyanide salt (hexacyanoferrate (II)) of alkaline metal or monovalent cation is added to the liquor that contains the dissolved vanadium in sulfuric acid, instead of the ferricyanide salt (hexacyanoferrate (III)) of alkaline metal or monovalent cation, as required by the electrodeposition procedure. The iron salt where this element exhibits its lower oxidation state is therefore directly added to the acid solution or liquor.

[0020] A precipitate is immediately formed with addition of the iron (II) salt to the vanadium containing solution, when sulfuric is the only mineral acid present. The composition of this solid corresponds to vanadyl and alkaline metal or monovalent cation ferrocyanide. Therefore, chemical separation of the dissolved vanadium can be carried out without passing electrical charges through solution.

[0021] When inorganic acids different from sulfuric are dissolved in vanadium containing solutions or liquors, for example nitric acid, the ferrocyanide salt of the alkaline metal or monovalent cation decomposes with the acid and the formation of a vanadyl complex becomes impossible. In this case, separation of vanadium from solution is only possible with application of electric charges as previously described.

EXAMPLE NO. 1

[0022] The vanadium dissolved in 150 ml of a solution containing 0.01 M vanadium pentoxide (0.1% dissolved vanadium), 0.013 M potassium ferricyanide (0.43% potassium ferricyanide) and 3.6 M sulfuric acid (35% sulfuric acid), was recovered passing electric charge through two platinum mesh electrodes of 86 cm² each.

[0023] A mixing rod was used to improve mass transport to the platinum mesh surfaces, in order to increase the electrodeposition efficiency. An electric current of 10 mA was applied for 3.1 hours, for a charge density of 3.8 C/cm².

[0024] When the electrochemical experiment was completed, the solution was filtered in order to collect the suspended solid, which was subsequently dried and weighted. The weight gain of the platinum mesh cathode, where electrodeposition of a green compound occurred, was also determined. The sum of both quantities, the filtered solid and the deposited compound, which corresponds to the total quantity of compound formed, was of 0.85 g, of which 0.14 g corresponded to vanadium.

[0025] In the experiment described above the quantity of vanadium initially present in solution was 0.15 g; thus the percentage of recovered vanadium was estimated relating the quantity of recovered metal to that initially contained in solution. In this sulfuric acid solution containing vanadium, the percentage of vanadium recovered was 93%.

[0026] In an experiment otherwise identical to that described above, except for longer electrodeposition time, with passage of 20 C/cm² of electrical charge, 0.89 g of compound were obtained. This corresponds to 0.15 g of vanadium in the solid obtained, for essentially 100% of recovery of the vanadium contained in solution.

EXAMPLE NO. 2

[0027] Vanadyl and potassium ferrocyanide was synthesized mixing equal volumes of an aqueous solution of 0.02 M vanadium pentoxide (0.20% of dissolved vanadium) and 3.6 M sulfuric acid (35% of sulfuric acid), with another solution containing 0.026 M potassium ferrocyanide (0.96% ferricyanide) and 3.6 M sulfuric acid. Precipitation of the vanadyl and potassium ferrocyanide compound occurred instantaneously when both solutions came in contact.

[0028] The solid formed was filtered, dried and weighted, and its composition was determined dissolving a known quantity of the solid in a concentrated inorganic acid, analyzing the elemental composition of this solution using Inductively Coupled Plasma (ICP) spectroscopy. The results indicated that the compound corresponds to the molecular formula K₂(VO)₃[Fe(CN)₆]₂15H₂O. The resulting yield of this reaction was of 98%; thus, 98% of the vanadium initially dissolved in solution was successfully recovered. 

What is meant to be protected is:
 1. A process that separates the vanadium contained in inorganic acid solutions which consist in: a. Adding a ferricyanide salt (hexacyanoferrate (III)) of alkaline metal or monovalent cation to the acid solution, in a proportion determined by the quantity of dissolved and to be recovered vanadium, in a minimal relation of at least two moles of iron in the salt for each three moles of vanadium contained in solution. It is also possible to add an excess of iron compound without affecting the percentage of recovery. b. Electrodepositing a solid compound of vanadyl and alkaline metal or monovalent cation ferrocyanide on a piece of conducting material immersed in the solution described in ‘a’. c. Electrodeposition is done applying a cathodic current density higher than 0.52 mA/cm², or a constant potential more negative than 0.85 V with respect to the saturated calomel electrode, to a piece of conducting material. d. The electrodeposition charge described in ‘b’ and ‘c’ is determined by the quantity of vanadium dissolved in the solution described in ‘a’ using the following relation: 36.8 C/cm² for each vanadium gram to be recovered. e. The solid builds up on the piece of conducting material and eventually, by the effects of gravity, detaches from the conducting material and remains suspended in solution. Then the solid suspended in the solution is separated with some physical method like filtration or centrifugation. Because of its low adherence, the material that has deposited over the conducting material is recovered by softly scraping the surface.
 2. A process that recovers vanadium dissolved in solutions or liquors of sulfuric acid which consists in: a. Adding a ferrocyanide salt (hexacyanoferrate (II)) of alkaline metal or monovalent cation to the acid solution or liquor that contains dissolved vanadium, in a proportion determined by the quantity of dissolved vanadium, and with a minimal relation of at least two moles of iron contained in the salt for each three moles of vanadium in solution. It is also possible to add an excess of the iron compound without affecting the percentage of recovery. b. Separating from solution or liquor the solid formed in the numeral ‘2 a’, vanadyl and alkaline metal or monovalent cation ferrocyanide, by means of a physical method such as filtration, centrifugation, decantation, etc.
 3. A process according to claims ‘1’ and ‘2,’ where the acid solution comes from the processing of a carbonaceous material.
 4. A process according to claims ‘1’ and ‘2’, where the carbonaceous material described in claim ‘3’ can be: crude oil and its fractions or residues originated from refining processes or treatments of crude oil, cokes, mineral coal and bitumen sands.
 5. A process according to claims ‘1’ and ‘2’ where the acid solution can be concentrated or dilute.
 6. A process that according to claim ‘1’ where the acid solution is selected among the following group of acids: sulfuric, nitric, perchloric, hydrochloric, phosphoric and hydrofluoric or a mixture of them. 